Electric motors have been widely used in refrigerant systems to drive compressors, fans, pumps, and various other components. As is known, in a basic refrigerant system, a compressor compresses a refrigerant, which is then sent to a first heat exchanger (usually a condenser or a gas cooler). After exiting the first heat exchanger, the refrigerant is then passed through an expansion device where the temperature of the refrigerant drops below the temperature of the air to be cooled and delivered to a climate-controlled environment. The refrigerant is then sent through second heat exchanger. Typically this second heat exchanger is an evaporator where the refrigerant absorbs the heat from the air (cooling the air), evaporates, and reenters the compressor.
In order to better control a refrigerant system and enhance the system efficiency, variable speed electric motors have increasingly been used in such systems. Variable speed drives provide a designer with enhanced flexibility in system operation and control. For a standard electric motor, the speed at which the motor operates is a function of an input frequency and the number of poles in the motor. Therefore, to vary the speed at which the motor drives an associated component of the refrigerant system, one can vary the input frequency of the electric motor to subsequently allow the motor to drive a component at a different speed. Thus, variable speed motors and associated driven equipment of the refrigerant system can operate across a wide spectrum of operational frequencies. A control for the variable speed motor may change the operational frequency as conditions or thermal load demands faced by the refrigerant system change. Typically, the variable speed motor starts from a frequency of zero and is ramped up toward a desired operational frequency. Thus, the frequency advances from zero upwardly to a set point operational frequency, which may be selected to achieve a desired cooling capacity, etc. Further, at shutdown, the frequency decreases from that operational frequency back towards zero.
A problem with these systems, however, is that certain operational frequencies create undesirable conditions such as mechanical and acoustic resonance, which may cause noise and excessive vibration in the components of a refrigeration system. The above-described systems, with the motor frequencies starting from zero and advancing upwardly towards the desired operational frequency, may pass through these resonance frequencies both at start-up and shutdown. Also, as the control changes frequencies during operation to satisfy external thermal load demands, it may sometimes move the electric motor operation to one of the resonance frequency zones that should be avoided.
This is undesirable, as excessive vibration, noise and pulsations may occur and result in damage of the refrigerant system components. The system resonance frequencies can also be excited by multiples of motor running speed frequencies, or by the running frequencies (or their multiples) of the driven equipment itself. It should be pointed out that the equipment running speed frequency can be different than that of the motor, if for example the driven equipment is attached to the motor via a gearbox, pulley or other similar means.
Some systems have attempted to overcome this problem by using stepless control and operating at these undesirable frequencies for a very limited time so as to avoid the resonance as much as possible. However, the methods used have not been able to entirely avoid the undesirable frequencies.